Patent GB-716712-A (GB)
Title: Improvements in or relating to demodulating circuits for frequency-modulated oscillations
IPC: H03G11/06
Filed: 1951-03-22
Abstract: 716,712. Frequency modulation. PHILIPS ELECTRICAL INDUSTRIES, Ltd. March 22, 1951 [March 29, 1950], No. 6894/51. Addition to 636,375. Class 40 (5) Demodulation of frequency modulated oscillations with simultaneous suppression of any amplitude modulation is effected by a circuit comprising quadripoles with common or coupled inputs and frequency dependent transfer impedances Z1, Z2 in the output circuit of an amplifying and amplitude modulating valve 1 to which a modulating voltage is retroactively applied from the rectified output of one quadripole Z2. The arrangement is such that if i is the output current of the valve and Z2 the transfer impedance of the quadripole Z2, the voltage |iZ 2 | is maintained constant as Z2 varies in accordance with the frequency excursions, i.e. i is amplitude modulated inversely proportionally to the variation in Z2. To this end Z2 may be off-tuned to one side of the mean frequency of the FM waves and the demodulated output is derived by a demodulator D, detecting the amplitude variations across the impedance Z1. According to the invention the expression if the demodulated output is derived in push-pull across a pair of impedances Z1, Zl', in lieu of Z1, is made a linear function of the (angular) frequency # or of where # 0  is the mean frequency. In Fig. 2 (not shown) Z1 comprises two coupled circuits each tuned to the mean frequency # 0 , second harmonic distortion being avoided by making the coupling just less than critical, and Z2 is a circuit offtuned from the mean frequency, in series with Z1. An adjustable low resistance may be in series with Z1 and Z2 to eliminate second harmonic distortion and third harmonic distortion may in this case also be eliminated by overcoupling the tuned circuits of Z1. Alternatively (Fig. 6) Z1 may be a resistor r and Z2 an off-tuned circuit transformer coupled to the output, r in the case of equal transformer ratios being equal to R2 the resonance resistance of Z2, and as shown Z2 may include a portion of r. Alternatively Z2 may be the sum or difference of the impedances across the primary and secondary of two critically coupled circuits each tuned to # 0 , being thereby equivalent to the impedance across a detuned circuit. In other modifications, Fig. 7 (not shown) and Fig. 8, Z1 and Z2 use coupled circuits 11, 12 each tuned to the mean frequency # 0 , Z2 being the transfer impedance of the circuits and Z1 the sum or difference of impedances across the primary and secondary circuits. In Fig. 8, the circuits 11, 12 are cross-coupled by a capacitor connected to a centre-tap on the primary, a diode D2 producing an amplitude modulation control voltage amc from across the transfer impedance Z2 which after reflex amplification is derived across circuit 28 and applied through a capacitor to a preceding variable-Á valve. Diodes 24, 23 are also connected between opposite sides of the timed circuit, where the transfer impedances are Z1 and Z1<SP>1</SP> respectively, to opposite sides of the low frequency output circuit 25. In this case is made linearly proportional to An AVC voltage is also derived as shown from diode D2. Specifications 636,375, 646,997, 659,598, 675,400, 676,541, 676,899 are referred to.